Generally, in the prior art, an additional reference voltage generation circuit is required to control the output voltage of the negative charge pump. The reference voltage generated from the reference voltage generation circuit is compared with the output voltage of the negative charge pump. When the output voltage of the negative charge pump reaches at a desired voltage level, a control signal is generated. The control signal is feedbacked into the reference voltage generation circuit for driving the negative charge pump and so controls the reference voltage generation circuit. As such, the output voltage of the negative charge pump can be controlled.
In the above prior art, an additional reference voltage generation circuit, capacitors, and means for feedbacking a signal therefrom into the reference voltage generation circuit are used to control the output voltage of the negative charge pump, resulting in a complication of the circuit and an increase area of the chip, and these analog circuits cause a characteristic variation to depend on a process and a temperature. As a result, it is difficult to exactly control the output voltage of the negative charge pump. If such an incorrect voltage is used in an analog circuit, there is a problem which could not remove data stored in a memory cell completely.